FIG. 10 shows an example of conventional servo motor system consisting of a servo motor unit 1, a servo driver unit 2, power lines L for supplying controlled electric power from the servo driver unit 2 to the servo motor unit 1, and signal transmission lines 3 for supplying signals related to the state of the servo motor unit 1 from the servo motor unit 1 to the servo driver unit 2, and FIG. 11 shows the changes in the signals supplied from the servo motor unit 1 to the servo driver unit 2.
The servo motor unit 1 comprises a servo motor 4, a pole sensor 5 and a rotary encoder 6 incorporated in the servo motor 4, a signal transmission circuit unit 7, and a temperature sensor 8 for detecting any abnormal temperature rise in the servo motor unit 1. The signal transmission circuit unit 7 comprises three line drivers 9 for sending out a Pu-phase signal, a Pv-phase signal and a Pw-phase signal obtained by the pole sensor 5, and additional three line drivers 10 for sending out a Z-phase signal, an A-phase signal and a B-phase signal obtained by the rotary encoder 6. These line drivers 9 and 10 are connected to associated line receivers 12 and 13 provided in the servo driver unit 2 via signal lines 14 through 25 constructed as balanced signal transmission lines. The signal lines 14 through 25 include six signal lines 14 through 19, or the +Pu, -Pu, +Pv, -Pv, +Pw and -Pw lines, for transmitting the Pu-phase, Pv-phase and Pw-phase signals, additional six signal lines 20 through 25, or the +X, -X, +Y, -Y, +Z and -Z lines, for transmitting the Z-phase, A-phase and B-phase signals, a pair of lines 26 and 27, or the T.sub.H1 and T.sub.H2 lines, for transmitting a temperature signal from the temperature sensor 8, and lines 28 through 30, or GND, +5V and 0V lines, serving as +5, +0 and ground lines. In FIG. 10, R, S, T and E denote power lines for supplying three-phase AC power.
Thus, according to this three-phase servo motor system, 17 lines are required in addition to the power lines L, and a considerable amount of work is required to perform necessary wire connections. Furthermore, each wire for the 17 signal lines must have a small diameter in order to control the overall diameter of the cable incorporating these signal lines, and this increases the risk of wire breakage. Also, the costs required for connectors contributes to the increase in the overall cost of the servo motor system.
Suppose that the rotary encoder 6 produces 5,000 pulses as A-phase and B-phase signals for each revolution, and the servo motor 4 rotates at 1,800 rpm. The frequency of the A-phase and B-phase signals is 150 kHz, and the phase different between the A-phase and B-phase signals is 1 microseconds in the worst case. Therefore, the quality of the signal lines must be correspondingly high to be able to transmit signals of such high speed. Therefore, it may be conceivable in theory to transmit various signals as a serial signal transmission process through signal lines of a smaller number, but the need for extremely high speed signal transmission lines has prevented it from being accomplished.